The durability and service performance of an asphalt roadway can be affected by a number of variables, including the asphalt mix composition that is used, the construction techniques employed in building the roadway, the overall weight and the axle weight of the vehicles using the roadway, the number and speed of the vehicles and the temperature and other environmental factors under which the roadway is used. Most of these various factors are beyond the control of the road designer. Furthermore, as traffic has increased on the nation's highways and as high-pressure radial tires have become more commonly used on heavy trucks, wear and even deterioration of the roadways has accelerated. Many of the nation's roads have suffered significant rutting damage, and are in need of repair.
Rutting is a manifestation of differential surface deformation in the wheel paths of a roadway which results from selective densification and shear deformation. Generally, the amount of pavement rutting depends, at least in part, on the traffic count and loads placed on the roadway and the distribution of such loads across the roadway. It will also be affected by the stresses introduced into the pavement system, and by the permanent strains induced as a result of these stresses. Rutting reduces road serviceability and driving comfort and will reduce the service life of a roadway. In addition, rutting may also contribute to safety hazards that may arise from an accumulation of water in the rutting paths. Such accumulation may lead to hydroplaning, or in appropriate weather conditions, icing.
One means of repairing a rutted roadway is to overlay the existing pavement with a new layer (often called a levelling course) of asphalt concrete. However, this method of repair generally results in the application of insufficient quantities of paving material in the rutted areas, because the overlayment will be applied at the same rate per unit of roadway width in the rutted areas (which have a higher surface area across the width) as in the undamaged areas. The resulting reduced density in the overlayment of the previously rutted areas will lead to renewed rutting in the new pavement in relatively short order. However, by milling or cold planing the surface of the damaged asphalt concrete pavement to a flat surface, the ruts will be eliminated and the new pavement will have a uniform density across the entire width of the roadway. In addition, a repaving technique that includes milling a thickness of old pavement and replacing it with an equivalent thickness of new pavement will return the elevation of the roadway to its initial level, whereas the placement of a levelling course atop damaged pavement will tend to raise the surface of the roadway or some portion thereof above its original elevation. This can require the raising of road shoulders, guardrails and manhole covers and the adjustment of overpass clearances, all of which is unnecessary if a proper milling technique is employed. A use of milling prior to repaving can also permit ready establishment of the proper road grade and slope, and thereby avoid drainage and safety problems. Furthermore, milling typically provides a rough surface that readily accepts and bonds with the new asphalt overlayment. Finally, milling can provide raw material that can be reclaimed for use in asphalt concrete production. Milling can also be utilized to remove the damaged upper surface of a ready-mix concrete roadway, preparatory to its being repaired or repaved with a layer of asphalt concrete.
Milling machines are typically wheeled or track-mounted vehicles that are provided with a rotating drum that includes a plurality of cutting teeth. The drum is mounted on the frame of the machine and adapted to be lowered into contact with the road surface and rotated about a fixed horizontal axis so as to cut into the surface to a desired depth as the machine is advanced along the roadway. Power for rotation of the drum is usually provided by the drive engine for the machine. Generally, the machine also includes a conveyor system that is designed to carry the milled material that has been cut from the roadway by the rotating drum to a location in front of the machine for deposit into a truck for removal from the site. Such machines are designed to cut into the pavement surface to a depth of eight inches (20.32 cm) or more, and for a width of up to 13 feet (3.96 m). Consequently, they are necessarily quite massive and powerful machines. It is not uncommon for a milling machine that is adapted to mill a full-lane width (13 feet) of a roadway to require an engine having up to 1200 hp (895 kW) of power. In addition, the conventional milling process puts considerable physical stress on the cutting teeth and their mounting means on the drum. Cutting tooth wear and breakage is a serious problem in the operation of conventional milling machines. Such machines are therefore somewhat expensive to operate and maintain, and consequently, various methods and devices have been developed to improve efficiencies of operation of the milling machine.
Most such improvements have related to the arrangement and configuration of the cutting teeth on the milling drum. U.S. Pat. No. 5,078,540 of Jakob et al. and U.S. Pat. No. 5,505,598 of Murray describe examples of such improved milling drum assemblies.
Other improvements have related to the means by which the machine cuts the pavement or other operative features of the machine. One such improved means of operation is described in U.S. Pat. No. 3,072,391 of McDarrah. This patent describes a milling machine which includes a small cutter drum having a plurality of cutter elements supported thereon at one end so as to be capable of swinging freely about a plurality of shafts that are parallel to the axis of rotation of the drum. As the drum rotates, the centrifugal force on each cutter element is such that each element extends substantially radially of the axis of rotation of the drum to provide both a cutting and impact action on the roadway. However, the cutting elements of the McDarrah machine are not as efficient or as easily replaceable as are the cutting teeth of the conventional milling machine.
It would be desirable if an improved milling machine could be developed that would increase the efficiency of operation of the machine without substantially increasing its weight, capital cost or operating cost. It would also be desirable if such an improved machine could be developed that would employ the type of cutting teeth that are commonly in use in modern milling machines. It would also be desirable if such a machine could be developed that would permit mounting of such teeth or cutting tools in the manner commonly in use.